Target structure for color television display tubes



y 1957 V c. s. NUNAN 2,801,355

TARGET STRUCTURE FOR coma TELEVISION DISPLAY TUBES Filed May 10, 1954 4Sheets-Sheet l Arrow 5Y5 y 0, 1957 c. s. NUNAN 2,801,355

TARGET STRUCTURE FOR COLOR TELEVISION DISPLAY TUBES Filed May 10. 1954 4Sheets-Sheet 2 IN VEN TOR. (24/6 5. A u/m/v July so, 1957 v c. s. NUNAN2,80 5

TARGET STRUCTURE FOR COLOR TELEVISION DISPLAY TUBES Filed May 10, 1954 v4 Sheets-Sheet 3 IN V EN TOR. (04/6 5. A/uA/A/v gamma lrropmi/i C. S.NUNAN July 30, 1957 TARGET STRUCTURE FOR COLOR TELEVISION DISPLAY TUBESFiled May 10. 1954 4 Sheets-Sheet 4 United TARGET STRUCTURE FORYCGLORTELEVISION DISPLAY TUBES Application May 10, 1954, Serial No. 428,622

8 Claims. (Cl. 313-48) This invention relates to target and colorcontrol structures for cathode-ray tubes designed for display oftelevision images in substantially natural color. Specifically, itrelates to such tubes of the type wherein the cathode-ray beam whichtraces the image is refocused in the'space adjacent to the displayscreen so that a beam, which is of the order of magnitude incross-sectional area of one elemental area or image point of the picturedisplayed, is concentrated on a sub-area of smaller size. The sub-areaon which the beam is focused is occupied by -a phosphor emissive oflight of one component color of a combination additive to produce White.in the region of the display screen has been termed and will be referredto herein as post deflection-focusing. Where post deflection focusing isused a supplemental electrode structure or grid, apertured to permit thepassage of electrons of the cathode-ray beam, is mounted close to Suchrefocusing of the beam Patent and generally parallel to the displayscreenand there is I also provided an auxiliary, electron-permeableelectrode in the same general region. The structure is such-that, when asuitable potential difference is applied between the apertured grid andthe electron permeable auxiliary electrode, there is formed amultiplicity of electron lenses distributed over the target area andeach having an aperture of the order of magnitude of one pictureelement. The screen is provided with sub-areas of phosphors grouped in arepeating pattern across the screen, each group comprising phosphors.emissive of all of the.component colors of the additive system employedand'each group electro-optically alined with a corresponding aperture ofthe grid. The particular "color displayed at any instant isdeterminedeither by the angle of incidence of the beam at the grid or bymicrodeflection of the beam ac complished in the plane of thegrid-itself. In the latter case the grid is formed out of twosetsof'interleaved electrodes. Potential differences between theelectrodes of the two sets establish fields which deflect the electronspassing through, the mean potential of the two sets being maintained,with respect to the auxiliaryclectrode, such as to effect the desiredlens action and bring the beam'to a refocus upon the proper color.

In tubes of the post-deflection focusing type the most convenient formto manufacture is one wherein the auxiliary electron permeable electrodeis a thin conducting film, usually of aluminum, depositedonthesurfa-ceof the display screen itself. This not only has theadvantage of simplicity of construction but reduces the power requiredto effect the scanning deflection of the beam when producing an image ofa given brilliance, since the same potential difference which does thepost-deflection focusing also;accelerates the electronsand impartsgreater energy to them as they arrive at the-screen.

When the system of focusing thus described isemployed there are threefactors which must betaken intopaccount if satisfactory pictures are, tobe produced. The=acceleration imparted. to the electrons betweenthe-grid? andthe display screen deflects themfrom theirstraight'linettravel,

Patented July 30, 19157 the amount of deviation from, the straight pathbecoming greater as the scanning angle is increased; accordingly, thephosphor areas must be alined with their corresponding grid' apertures.electro-optically rather than structurally, to take account of thecurvature of their paths in the grid s'creen region. The. principlesunderlying such electro-optical alinement are set forth in the copendingapplication of Ernest 0. Lawrence, Serial No. 399,754, filedDecember 22,1953. The focusing effect of the gridto-scree'n field also varies withthe angle of deflection, the focusingeffect. being. stronger withgreater angles offincidence of'the beam at the grid. Hence if the beamis brought to a sharp focus at the center of the screen it will beoverfocused and produce a larger focal spot as it "is deflected towardthe edges of the screen; conversely, sharp focus at the screen edgeswillrcsult in less than complete convergence at the center of the screenand hence a. compromise value of focusing voltage is frequently used inorder to obtain a minimum average spot size. Finally, wheremicro-deflection of the beam is used to accomplish color switching, thesensitivity of the beam to a given deflecting voltage also increaseswith the angle of incidence of the beam at the grid. This'latter effectcan, be 'compensateito a large degree, by varying the shape of thephosphor areas as is disclosed in thecopending application of Ernest 0.Lawrence,- Serial-No. 399,753, also filed December 22, 1953.

The varying sensitivity to micro-deflection at the grid can also becompensated by imparting a curvature to the screen-or the. grid, ,sothat electrons travelling in theinterspace between screen and grid atthe center of the screen have alonger distance to travel after themicro-deflecting impulse has been applied than those traversing the gridstructure at the edge of the screen. By proper. computation of the gridor screen curvature the displacement of the point ofimpact by a givenapplieddefleeting impulse becomes. substantially uniform. The problem ofelectrooptical alinement of the grid apertures and phosphor groups canbe solved in this latter case by applying the same principles as whereuniformgrid-to-screen spacing is. employed. The expedient has, however,no effect on the defocusingofthe beam by varying angles of incidence.

Thebroadpurpose of the present invention is to provide atargetstructure, including display screen and electron lenssystem, whichpermits simultaneous solution of the-three problems of electro-opticalalinement, uniform focusing-throughout the display area anduniform-microdeflection sensitivity. Other objects and advantagesaccomplished by the invention are the provision of a structure whichpermits a maximum duty cycle to be utilized in the display of eithercolor or monochrome television images, to provide a target structurewhich will give uniform spot size and definition, both as to color andmonochrome, in all parts of the screen; to provide a structure whereinthe display screen may be formed upon the viewing window of the tubeitself, instead of upon an auxiliary base positioned within the tube andbehind thewindow, the screen having sulficient strengthto withstandatmospheric pressure even in large sized tubes, and, by virtue oftheimposition of the screen upon the window, avoiding the transmissionlosses through multiple glass surfaces at which reflection losses canoccur; to provide a type of target' structure which may, in anapproximate, slightly modified form, he cheaply constructed and readily.computed and still offer to a high degree the advantages of nearlyuniform focusing and micro-deflection sensitivity; and to provide atarget structure which, in either its substantially exact or moreapproximate form, is fully capable of commercial production atreasonable cost.

In the type of electron lens or aggregate of a multiplicity' of electronlenses formed between an apertured,

grid and the substantially equi-potential plane formed by a conductingfilm on the screen or other electron permeable electrode, the positionof the focal point or maximum convergence of the electron beam isdetermined wholly by the ratio of the accelerating voltages between thebeam source and the grid to that between the grid and the equi-potentialplane, the shape of the grid apertures, and the angle of incidence ofthe beam at the grid, and is substantially independentof the distancebetween grid and the equi-potential surface. In accordance with thepresent invention an additional electrode structure is utilized, thisstructure comprising a I second grid mounted between the first grid andthe screen.

face thus described lying wholly on the side of the second I the secondgrid is also preferably comprised of wires tightly stretched across thedisplay screen in a direction normal to that of the electrodescomprising the first grid.

When so arranged the wires of the second grid act to form slightlydiverging lenses, increasing in some degree the size of the focal spotin a dimension parallel to the phosphor strips, but have. no defocusingeffect in the direction of width of the strips. With other types offocusing and display screen, the effect of the second .grid

is to increase slightly the focal length of each of the componentelectron lenses of the aggregate. Moreover, when the invention is usedin tubes of the type using linear grid electrodes and strip phosphors arelatively close approximation to the desired uniform focusing anduniform deflection sensitivity can be attained by using a screen whichis curved in only one dimension; i. e., is a section of a right cylinderwhose axis lies in a plane at right angles to the planes of the gridelectrodes. .When used in this modified form in a tube wherein themaximum scanning deflection is 72 the deflection sensitivity may be heldconstant to within :2.2% over the entire screen area and the maximumspot size will be only 2.2% of the aperture width greater than theminimum;

in practice this means the width of the spot will vary between 3 milsand 3% mils; the corresponding variations in a like tube using a planarscreen and grid are 11%. In the case of a tube using a 90 scanningdeflection, the other structural parameters being the same, thevariation in spot width will be between a minimum of approximately 3mils and a maximum of approximately 4 and the micro-deflectionsensitivity can be held constant to within i-3% throughout the screen.Using a flat grid and screen the variation in deflection sensitivity isabout i20% and the variation in spot sizeis over 100%, i. e., between aminimum of 3 mils and a maxi- -mum of about 7 mils.

It will thus be seen that very large advantages accrue from even themodified approximate form of the invention.

Referring to the drawings: e

Fig. l is a diagrammatic view, in cross-section, of a cathode-raycolor-television display tube embodying the present invention;

Fig. 2 is a fragmentary cross-sectional view of a disicate the order ofthis arrangement.

lying the phosphor coating, there is a thin conducting 'and thereforethe device is fully operative even though equivalent thereto.

play screen adapted for use in a tube of the type diagrammed in Fig. 1;

Fig. 3 is an exaggerated perspective view of a viewing screen, shaped inaccordance with the present invention, and adapted to give completefocusing and deflection sensitivity correction over the entire screen;

Fig. 4 is a similar view of a cylindrical screen, adapted to giveapproximately uniform focusing and deflection sensitivity throughout thescreen area; and

Fig. 5 is a family of curves illustrating, in terms of the spacingbetween the grids of the tubes of this invention, the screen curvaturein various planes parallel to the conductors of a linear-elementdeflecting grid.

With the exception of the target structure, comprising the displayscreen and the grids associated therewith, the tubes in which thepresent invention is incorporated may be conventional in structure,comprising an evacuated envelope of generally funnel-shaped form, whichmay be either of glass or of glass and steel construction. In thepresent instance the latter type of construction is shown, the tubecomprising a cylindrical glass neck 1 wherein is mounted an electron gunhaving an electron emitting cathode 3, a control grid 5, a first anode 7and a second anode 9, with the usual connections for applying suitableelectrical potentials to each of these elements. The second anode ispreferably connected either to the funnelshaped metal shell 11 whichconstitutes the body of the tube, or, if the tube be of all glassconstruction, to a conductive coating deposited upon its inner surface.The large end of the funnel-shaped body of the tube is closed by aviewing window 13, which, in the case of the tube illustrated in Fig. 1constitutes the base upon which the light emitting phosphors of thedisplay screen are deposited. It will be noted that the window isillustrated as curved, i. e., convex outwardly. The exact form of thiscurvature will be discussed hereinafter.

Fig. 2 shows, upon an enlarged scale, a small crosssection of the window13 and the phosphor screen thereon. Deposited upon the base 13 arestrips of phosphors which are emissive, upon electron impact, of lightof different component colors additive to form white. Each group ofstrips extends across substantially the entire face of the screen in onedimension, and its width, which is uniform or very nearly uniformthroughout its length, is of the order of magnitude of one element orpicture 'point of the television images to be displayed thereon.

In the type of tube illustrated the strips are deposited in a repeatingpattern in the order red, green, blue, green, red, etc., and in thediagram of Fig. 2 the various strips are designated as the initialletters R, G, and B, to indi- Preferably, overfilm 15, which ispermeable to electrons. Such films are well known in the art and theyare usually of aluminum. They serve the triple purpose of establishing adefinite potential for the screen, of reflecting back, out through thewindow, light which would otherwise be radiated back into the body ofthe tube and lost, and of suppressing, to a considerable degree, thesecondary emission of electrons. This conducting film is not, however,an absolute essential in the operation of the tube, since under thebombardment of the cathode rays the screen will emit secondary electronsuntil it reaches an equilibrium potential which is only a few voltsnegative to certain portions of the electrode structure of the targetthe film 15 may be omitted.

Stretched across the viewing window or screen and in a directionsubstantially normal to the phosphor strips is a grid 17 of fine wiresor other linear conductors The spacing of the conductors of the grid 17(hereinafter referred to as the second grid, even though it is the firstto be described) is not critical, although preferably it is of the sameorder of magnitude as the spacing of the centers of successive greenstrips tin thewscreen; Asawill be shown hereinafter it isaconvenientthat the conductors of the grid l7contactz the edges of the screen,although .this,.;too, isnot essentialto the operation of the tube aswill be set forth in detail hereinafter. It will be obvious froma-description of the construction that:the grid "17 is substantiallyplanar when considered as a :whole.

Mounted in a plane parallel to that-iofj the grid 17 is a second gridoflinear electrodes, 19 and 19', whichextend across the surface of thescreen in a direction substantially normal to the extension of theconductors of grid 17 and therefore substantially parallel totheaphosphor strips. The: conductors 19 and 19 alternate. The set ofconductors 19 is connected to a common lead 21 which is brought outthrough the envelope. Similarly conductors 19' are connected "to acommon lead 21 also brought out through the wall of the envelope. Thetwo sets of conductors are mutually insulated so that apotentialdifference can be established'between the two sets ofconductorsto effectmicro-deflection of'the beam.

The conductors 19 and.19' are so positioned that the center of theaperture'formed between each adjacent pair of electrodes iselectro-optically centered in front of the center of'a corresponding oneof the green emitting strips G; that is, they are so positioned thatwhenthe electronfor concentrating the beam from the electron gun and forscanning it over the surface of the target area, are employed. Sincesuch focusing and deflecting coils are conventional and are not a partof the present invention they are omitted for simplicity in the drawingand description.

As is usual in the operation of cathode-ray display tubes for televisionuse, the cathode 3 is operated at the lowest 'potential'in the system,and anodes 7 and 9 are progressively more positive. The two sets ofelectrodes 19.and 19, comprising the first grid, are preferably operatedat a potential of between 200 and 400 volts negative to that of thesecond electrode and the tube shell, assuming that "the latter is in theneighborhood of 5000 volts to 8000 volts positive with respect to thecathode. This, of course, refers to the mean potential of the two setsof electrodes; in operation an oscillating voltage of perhaps 400 voltspeak to peak is applied between the two sets of electrodes of-the grid.The second grid 17 is operated at a potential considerably higher thanthe mean potential of the first grid, and the conducting film 15 isconnected to and therefore operated at the same potential as the grid17.

The cathode-ray beam is deflected bidimensionally over the area of theviewing screen to trace a raster thereon in accordance with .usualtelevision practice, vertically at substantially 60 cycles per secondand horizontally at substantially 15,750 cycles per second in accordancewith present standards in transmission in the United States, or inaccordance with whatever other standards may be employed in the servicefor which the tube is to be used. Preferably the conductors 19 and 19'extend across the screen in the direction of the higher frequency orline deflection.

As a result of the voltages applied to the various electrodes of thetube the cathode-ray beam developed by the electron gun travels in asubstantially straight line from a center of deflection, the exactposition of which depends on the position of the deflecting coils, toreach the plane of the first grid 19-19 at varying angles of incidence.As the beam passes through the plane of the considered-overall, andthrough the major portionof-the distancebetwen the-two ,grids it issubstantially uniform and-in ardirection normal to the plane ofbothgrids; thereforeit-bends, :the beam so thatits angle of incidenceto-the-;second grid is less than that of its angle of incidence to-thefirst grid. In the immediate neighborhood of the grids thefield isconcentrated on the conductors-comprising them. Inzthe plane of thefirst grid this .field is directed away-from the wires, giving theelectrons constituting the-beam an impulse which tends to converge them.This impulse is in the direction normal to the wires and its effectis-to make the beam converge into a fine-line parallel to-the gridconductors. In a practical tube the. distance between the grids will beof the order of 10to 15 times .the-distance between thegrid conductors,andwith these proportions no material error is involvedin, consideringthat the entire impulsewhich causesthe. convergence of the beam isapplied to theplane of the grid itself. The same-holds true of the shapeof thefields .at,the,second grid .17, but here the direction of thefield issuchas to cause a divergence of the beam. Thisdivergence,however, doesnot affect the linear character of the focus imparted bythe first grid, since there,

e the screen is materially shorter than that betwen the grids,and-therefore the electrons have a shorter time in which to diverge,even at the center of thetube where the distance between the second gridand screen is greatest.

When there isa potential diiference between the two sets of electrodesof the first grid there issuperposed upon the two effects mentioned anadditional impulse which deflects-the entire beam toward the morepositive electrode of the pairforming the aperture through which thebeam enters, and if this-impulse is of the proper magnitude the beamwill be deflected from the green strip, upon which it was originallycentered, to fall upon either the red or blue strip as-the case may be.

It will be seen from the above that in the region between, the firstgrid and the screen, the electrons of the beam are subjected to threedifferent accelerations which divert them from their straight linepaths. The first of these is an acceleration directly toward the secondgrid and screen, resulting in a refraction of the beam as a wholeandcausing its center to fall on a point of the screen closer to the axisof the tube than it would if the field were not applied. The second isthe converging or focusing action of the second grid. The third is themicro-deflection of the beam which controls the color displayed by thetube and which will be referred to hereinafter as the color deflectionto distinguish-it from the scanning deflection.

In the copending applications of E. 0. Lawrence, Serial Nos. 399,753,and 399,754, cited above, equations are given showing the magnitude ofeach of these deflections, at various pointsof the screen, in tubeswhich are substantially similar to the one here described except for thefact that the screen is planar and the accelerating field is applieddirectly between the screen surface and the color deflecting grid. Itshould be ob vious that on the axis of the tube this accelerating field'has no refracting efi'ect, since the general direction of accelerationcoincides with the line along which the beam final velocity in thisdirection.

over the entire screen. "the screen as to accomplish all of thesepurposes will next the screen is lower and it is therefore subjected tothe accelerating field for a longer time and because the component ofvelocity added in the grid-to-screen' region, normal to the screen, is agreater proportion of its total The sensitivity'to focusing is alsogreater at greater angles of incidence, both because of the longer timetaken by the electrons in passing through the field where focusingdeflection takes place and because there is a longer time during whichthe electrons are traveling between grid and screen and the velocitycomponents causing the convergence is effective. Finally, thesensitivity to the color deflection increases with increased angle ofincidence for the same reason, and substantially to the same extent asdoes the focusing sensitivity.

As long as planar grids and screens are used, therefore, certainapproximations must be made to obtain the best over-all focus andover-all deflection sensitivity throughout the screen. As has been shownin the copending applications above mentioned, the focusing effeet issubstantially independent of the distance between grid and screen. If itwere possible, in a practical tube, to give the screen and grid both acurvature concentric about the center of scanning deflection, the angleof incidence of the beam would be a constant and uniformity of bothfocusing and color deflection would be achieved. With screens of thesize now demanded for television viewing, however, such concentricitywould involve either an unduly long tube or a screen so convex as tocause an apparent distortion of the image when viewed. Either the screenor the grid can be curved to give substantially uniform deflectionsensitivity, but in other than the concentric relationship this does nothelp defocusing.

In accordance with the present invention, however, using twosubstantially planar grids, and applying focusing potential between thetwo grids so that the beam is not over-focused at its maximum angle ofincidence, the screen can be given a curvature and the relationshipbetween the pitch of the grid wires and the width of *the phosphorstrips can be so adjusted that not only 'are the groups of phosphorstrips electro-optically alined with the corresponding apertures in thecolor control grid, but the screen lies in the focal surfaces of thevarious electron lenses forming the color grid and because of thesimilarity of the conditions for focusing and for color deflectionsensitivity, the latter is substantially uniform The conditions for soforming be discussed in detail.

Refraction is proportional to the square root of the potentials of thesevarious elements with respect to the cathode. The

potential of the first grid is so nearly that of the second anode of theelectron gun that the space between the gun and the first grid may beconsidered, without appreciable error, as an equi-potential space, andall lines of force originating on one grid may be considered asterminating upon the other, since the space between the second grid andthe screen'is substantially equi-potential.

In this analysis, as'well as those which follow with respect to focusingand other deflection sensitivity, the following notation will be used:

Potential difierences between cathode and first grid=V1;

Component of angle of incidence of beam at first grid in plane parallelto grid electrodes=a;

Component of angle of incidence in plane normal to wires of firstgrid=;3; t 1

Total angle of incidence of beam at first grid=6 (tan 0= tan a+tan i3);

Displacement of beam electrons in the )3 plane (i. e., in directionnormal to wire of first grid) by fields efi'ective in the grid-screenregion=y; subscripts indicate the nature of the displacementsconsidered.

Because of the fact that displacement of the point of beam impact in thedirection parallel to the electrodes of the first grid is so small as tocause no visible distortion of the picture and, furthermore, becausedeflection in the latter direction causes no change in color displayedon the screen, components of deflection in the latter direction need notbe considered in the design of equipment in accordance with the presentinvention.

Using the same type of derivation as that given in the copendingLawrence applications above identified, it can be shown that thedisplacement y of the center of the beam under the influence of theaccelerating field alone is I 2D tan 6 F tan [3 1+ /1+ksec. 0 1+kseo. 0

The quantity y is the distance in the direction normal to the electrodesof the first grid, between the foot of a perpendicular dropped from thecenter of one of the electrodes 19 or 19' and the center of acorresponding red or blue emitting phosphor strip, as the case may be,at a given point of incidence of the beam corresponding to a particularvalue of 0; or, stated otherwise, it is the distance measured in thesame direction between the foot of a perpendicular dropped from thecenter of the aperture between two electrodes 19 and 19 and the centerof the corresponding strip of green emitting phosphor. As in the case ofplanar grids and screens, the quantity y varies with both a and 5components of the angle 0. It results in screen dimensions which arevery slightly larger than the dimensions of the active portion of thegrid, but are smaller than the ratio of the distance between the centerof deflection and the first grid to the distance between the center ofdeflection and the screen. When the beam is deflected over a rectangularraster at the first grid the refraction causes a very slight barreldistortion of that raster as it appears on the screen, but thisdistortion is only of the order of one-tenth of one percent of thescreen dimension and is not perceptible at all in ordinary viewing. Inorder to meet the requirement of electro-optical alinement, each groupof strips must be located at a distance from the axis of the screenwhich exceeds by the amount y-, the

distance of the corresponding aperture of the grid from the grid axis.Since varies from point to point over the grid, the ratio of pitch ofthe grid wires to the phosphor spacings varies from the center of thesceen outwardly toward its edges, as is the case with the single planargrid and planar screen, and correction for the distortion and pitchchange can be accomplished in the same manner as is disclosed in theLawrence application, Serial No. 399,754, identified above. As is thecase with the single grid and planar screen, the change in ratio ofspacing can be accomplished either by varying the widths of the phosphorstrips or by varying the pitch of the grid electrodes, eithercontinuously or in zones, from the center of the screen out toward theedges. For the purpose of this invention it is of little practicalimportance whether the change of ratio is accomplished by varying thewidth of the groups of phosphor strips or by I varying .the pitchof the.grid conductors. Theoretically the former offers a slight advantage -inmantaining: constant color-deflection sensitivity, but the differencesare so small that in practice they'may be ignored.

Focusing,

Although. Equation .1 is given interms .ofithe. component angleofincidence B, .the actual point .of :impact of any. electron upon thescreen is related .to 'i'ts point :of entry at the grid. by itscomponent of velocity parallel to the grid plane, times the. timeinterval between the instant of passing through'the color controlgridand the impact upon the-screen; The forcesto which an elec- -tron issubjected in passing through the grid do not change itsi absolute.velocity but do change the-=rela'tionship between the components of thatvelocity parallel and normal respectively to the plane of the grid. Ineffect, therefore, the focusing and deflecting forces at-the gridchange: the point of impact of any individual electron in the samemanneras a change inthe-component'angleiof incidence :5. What the,changeinfiwill be-for agiven focusingoncolor deflecting impulse can be.computed as will be shown hereinafter. The efie'ct of such changes onthe position of the point of impact is, however, proportional toandtaking the derivitive of Equation 1 with respect to fi-gives Y 2 W i1+ /1+Ksec. 9 w l-t-Ksecfifi F tan a It has been shown in the Lawrenceapplications above referred to-that the convergence of the beam duetothe focusing-;fielcl,;fora grid of linear conductors, is equal to theright handquantity of Equation '2 multiplied by In the above-equation Ayrepresents the displacement .of the point of impact of electronsentering the aperture, and ye represents the distance, normal to thegrid electrodes, between the point of entry of the electrons into theaperture and the aperture center. It follows that when from the value of1, multiplied by the width of the aperture, plus this minimum widthof=practicalfocal newt-teammates verywlosel the actual width otnhefoca1.=spdt as':observedand measured.

In accordance with .the present invention awsuitable valueiswhosenfontheseparation -D betweenthe -first and second: grids, and-the screen issoiformed that over its surface -ther-valuevofi F, theseparation betweensecond grid. and i'screen, .isasu'ch ithat throughout the target areathe quantity isnsubstantially equal, to -;l. Since the valueof Dis amatter of choice, the quantity which is really of interest is theratioofF to D.-,- Therefore, setting For manufacturing reasons it isconvenient to make :the-second grid contact the edges of the screen, sothat at this :portion of -.the structure F=0. This is not a neces--sary:; condition for the practice of the invention, but it :doescombine maximum post-deflection acceleration, .maximum:sensitivity toboth scanning and color control deflections, andminimum bulk of thetarget structure, and is therefore to be preferred. Actually, however,the

screen can be formedito conform with the requirements of Equation 4 solong as the quantity F at its edges equal to or greater than 0.

In Fig. 5 there is given a series of curves in which the value of ascomputed from Equation 4, is: plotted for various values 'of the angle 0and its component [3. These curves are. plotted for tubes of twodifferent designs. In this figure, curves 25, 27, 29 and 31 are plottedwith respect to .ahtubeof. the deflection type, while curves 3'1, 33 and35 refer'to 72 tubes; i. e., with regard to the first set of curves themaximum value of 0, when the beam isdefiected to the corners of thedisplay screen, is taken at 45, while in the latter set of curves themaximum angle of 0 is'36.

In plotting all of these curves it has been assumed that thee1ectrodes-19, 19' extend in a direction parallel to the longerdimension of'the picture field and that the latter has the standardaspect ratio of 3 units vertical to 4 horizontal. Intheseplots, with theexception of curve 30, the'absci'ssas are given in terms of tan a andthe ordihatesin-terms of the ratio hasin each case been chosensubstantially to bring the minimum value of. F to 0. For the 90 tubethis value of K'is 1 .892, While for the 72 tube K=2.210.

' Curve 25 gives the values of F on the horizontal axis of the screen,where tan [3:0. Curve 27 is plotted for a 'value of tan 9:0.361. andcurve29 for a'zvalue ofitan 8:0.600, i. e., for the upper and loweredges.of the field where the angle )8 is a maximum. .1

The three curves for the 72 deflection tube have a generally similarsignificance. Curve 31 is drawn for tan 18:0; curve 33 for tan fi=0.308and curve 35 for the upper or lower edges of the field in a 72 .tubewhere tan 5:0.436.

The particular significance of curves 27 and 33 will be discussedhereinafter in connection with a modified form ofthe invention; for thepresent it is sufficient to note that these curves lie approximatelyequidistant from those representing the maximum and minimum concavity ofthe screen, along its axis and at its edges respectively.

Deflection sensitivity When a color changing potential is appliedbetween the conductors 19, 19"the effective change, AB in the angle )3can be expressed by-the equation:

where Va is the voltage applied between conductors 19, 19', s is thespacing of the wires or other electrodes of the grid and w is to thediameter of the grid wires, the other symbols used having the samesignificance as before. It may be noted in passing that the expressiongiven is actually for the change in sine 3 but because of the smallvalues of deflection angle used in practice, the angle and its sine areeffectively equal. Since Equation 2 gives the change in y for smallchanges in the value of 19, the relative displacement of the spot,

Itwill be seen that Equation 7 is similar in form to Equation 3, andexcept for coeflicientswhichare constant (or can be made constant) forany given tube, the equations are substantially identical. The ratio .Kis a constant and is the same for both equations. The deflecting voltageVa, is, at any instant, constant throughout the tube. The only quantitywhich may vary .with' angle and which is not identical in the twoequations is the denominator in the first term at the right of theequality in Equation 7. If the refraction correction is accomplished byvarying the width of the groups of phosphors, rather than the pitch ofthe grids, this term is also a constant over the surface of the screen.It follows that if the concavity of the screen is made such that thefocus is constant throughout, the deflection sensitivity will also beconstant. If the refraction correction is accomplished by varying thepitch of the grid wires, the widths of the phosphor groups beingmaintained a constant, there will be a slight difference in thesensitivity to deflection due to the changes in the quantity In one tubeof approximately the characteristics here described the quantity s, thespacing of the grid wires, averaged approximately 30 mils and thediameter of the grid wires w is 6 mils. The total difference in spacingof grid wires requisite to accomplish the necessary screen, wasapproximately one percent. The corresponding change in the quantity Zniii) was about 0.6%. The variation in sensitivity over the surface ofthe screen would therefore be in this same ratio, and the difference ordeparture from uniformity is sosmall in comparison with the ordinarymanufacturing tolerances that it can usually be neglected, irrespectiveof whether the ratio of change in pitch to phosphor spacing isaccomplished by varying pitch or phosphor spacing. It will always beless than the errors involved by making the spacing ratio changes inzones instead of continuously if this is the method adopted. As has beenshown in the previously mentioned Lawrence applications, displacementsof up to approximately one mil can easily be tolerated.

If the refraction correction, applied to maintain the apertures andphosphor groups in electro-optical alinement, is accomplished by varyingthe width of the phosphor groups, there is some theoretical advantage inmaintaining the width of the central strips (green in the example heregiven) constant and taking up the width variation in the strips whichare electro-optically centered under the grid wires, i. e., the red andblue emitting strips. Because the total variation is so small, however,this advantage is more theoretical than practical. The over-alldifference in phosphor group width as between center and edge of thescreen is, in the case mentioned, only about one percent, and if thedifference is applied equally to the red, green and blue strips, it isonly about one-tenth of a mil. Since this is of a lower order ofmagnitude than the mechanical precision which at present seems desirableto maintain in commercial manufacture, any errors due to the method inwhich the correction for refraction are applied can usually beneglected.

Actually, errors much greater than thosewhich have been discussedimmediately above can be tolerated in many cases. An examination of thecurves of Fig. 5 makes it readily apparent that the curvature of thescreen about an axis parallel to the wires of the color-control grid ismuch less than that about an axis at right angles thereto. Curve 30shows the variation of the ratio F/D along the vertical axis of thescreen of a 90 deflection tube, where the curvature is greatest. Thesubstantial coincidence of curves 25, 27 and 29 at the edge of thescreen, where tau oc=0.8 indicates that there is substantially novertical curvature of the focal surface in this portion of the field.The maximum degree of curvature in the vertical plane, for exact focus,is only about one-fourth as great as that in the horizontal plane.

Many of the advantages of the invention can therefore be obtained by theuse of a screen which is, in form, a right cylinder. In a 90 tube, thecontour of such a cylinder conforms in curvature to substantially thevalue shown in curve 27 or, in a 72 tube, with that indicated by curve33. These curves are drawn for values of the angle 5 where the amount ofdefocusing is substantially equal at the top and bottom and at thecenter of the vertical axis of the screen, the beam being underfocusedat the center and overfocused at the upper and lower edges. For a 90deflection tube this condition is substantially met where tan fi=0.40,for a 72 tube where tan 5:0.308.

Such a screen is shown in somewhat exaggerated perspective in Fig, 4. Ina 90 tube the screen surface will coincide exactly with the focalsurface only for deflection angles where tan fi=0.40, 18:21.89. Forother angles of scanning deflection both the deflection sensitivity andthe size of the focal spot will differ from the optimum. With thisarrangement there will be two horizontal scanning lines, correspondingto vertical deflections of i21.8 from.the horizontal axis of thescreen,where change in pitch ratio, from the center to the edge of the thefocus is practically perfect. It will be practically 2,801,5iii

perfect, also, at the left and right-hand edges of the screen. In thezones outside of thetwohorizontal lines of exact focusing,.thebeampassingthrough a:color.-grid aperture will pass through-thecross-overipoint which represents exact focus before reaching thescreen, and be slightly larger-than its minimum size by the; time itreaches the screen, whereas in the central zone between the two lines'ofperfect focus in1will..:not quite reach its minimum size where itimpacts thescreen. Similarly in the outer horizontal zone the deflectionsensitivity will be slightly greater than the average whereas in thecentral horizontalfzone the deflection sensitivity will be undertheaverage; The errorsmentioned will, of course, be greatest-*alongtheline markingthe vertical axis of the display screen. Actually themaximum increase in spot'width, using apertures in the first grid ofapproximately 30mi'ls wide, will be about 1 mil and the difference: indeflection: sensitivity about :3.6%. These figures comparewith increasesin spot size of over 4 mils (133%) and insensitivity of over 40% for a90 tube using a plane screen and a single grid. The variations in a 72tube will be much less. It will therefore be obvious that :even::theapproximately corrected form of the invention oflfers great practicaladvantages.

The use of a cylindrical' screen, giving anapproximate correction,hasthe advantage that it is somewhat easier to 'print the screenaccurately upon asurface which is curved in only-one dimension than itis to do so upon a screen curved Withrespect to both its axes. From themanufacturing point of view it is more ditficult to incorporate acylindrical screen as a portion of the envelope of the tube itself thanit is to do this with a screen that is curved in two dimensions,although the cylindrical screenmay bebowed at its ends, outside of thescreen area, and molded so that it' can be joined or sealed totheenvelope and willwithstand the atmospheric pressure. Cylindricalscreens may also, if desired, be mounted inside ofthe envelope in thesame manner as planar screens described in the previously identifiedLawrence applications.

In this connection it may be noted that although the curves of Fig.5show the relative values of the distance F'between the second grid andth'e screen, the abscissas and the ordinates are on greatly differentscales and therefore the curves do not give any visual picture or" theactual degree of curvature employed. In a tube using an eighteen inchdisplay screen, i. e., a tube wherein the surface on which the rasterdisplayed measures eighteen inches on the diagonal, the length of thedisplay surface is approximately 14% inches and its heightapproximately, inches. In a 90 tube having a display surface of thesedimensions the distance D may be 0.450 inch. In the case of a completelycorrected display surface in a tube using these parameters, the centerof the screen, as viewed from the electron gun side, will be 0.255 inchbehind the plane of the second grid, amounting to just a little over aquarter-inch bulge in the screen in. its span of approximately 14inches, whichis obviously very slight. With a 90 deflection tube thedeparture. of the fully-corrected screen from the cylindrical form .isonly a little over inch in a span of 10% inches. If, in a 90 deflectiontube, the approximate cylindrical correction is adopted. the bulge alongthe horizontal axis of the screen is 0.23 inch in the 14% inch spanandthe departure. from the true. focal surface-is at maximum only 0.025inch. In a'72 tube using the same inter-grid spacing D, the bulge of thedisplay screen for a fully corrected tube is only 0.153 inch maximum,and. that of the compromise cylindrical screen only 0.137 inch. Asviewed from the outside of the tube, therefore, the screen has enough ofa bulge to withstand atmospheric pressure but not enough so that itscurvature is'obtrusive or to cause an apparent distortion of thetelevision image because it is displayed on a curved surface.

The fact that the actual curvature-of the. field is slight, is importantforone other reason; The equations given in-this-specificationarebasedon animplicit assumption that the displacementof the points "ofelectron impact on the screenzis the same as it would be if the smallportion of the screen in the immediate area wherein the deflection takesplace were a flat surface, parallel to thetwo grids. As far as thefocusing deflection is concerned, this would involve substantially noerror, since the theoretical focus is a geometrical line and the actualfocal spot departs so:llittle from the theoretical value as to becompletely negligible. In the case of the color deflection, however, thecurvature could make a difference. Actually it does ,not,since thedeflection is in a direction normal to .theqgrids major curvature andtherefore the distance between second grid and screen does not changeappreciably-within the area through which thedeflection occurs.

The fact that the ,secondgrid: has a diverging or defocusing effect inthe direction normal to the focusing effect of the first grid has beenmentioned. In a tube wherein the electrodes of the first grid runparallel to the scanning line, as is. here described, the effect of thedefo'cusingaction-is to-lengthenthe focal line or spot in the directionofthe line scanning. If the primary focus of the beam, as accomplishedby the electron gun and the usual focusing; coil is poor, resulting inan initially over-large spot, this may be important, but underordinary.circumstances it is not. The amount of the defocusing action:which occurs is directly proportional to the distance F,- since thespace between the second grid and the screen is substantially aunipotential space as the second gridand screen are directly connected.Where the electrodes-of the second grid approach the screen veryclosely, some of the lines of force which the theory of operation;of'the invention assumes terminate on the second gridwill actuallyterminate on the screen itself; i. e., in the narrow zone where thedistance between second grid, and soreeniis small in comparison with theseparation of the second grid wires. In this area, however, the amountof convergence in the second gridscreen region, required to bring thebeam to the minimum focal size, is so small that it is unimportant. Inthe. zone where this is true the second grid causes substantially nodivergence in the plane normal to the second-grid wires. In the portionof the target area Where the-distance F is a maximum the increase in thelength of the focal line is less than 20% of the diameter of the beam asit enters the focusing structure, and it is to be remembered that thiscan be as small as is normally the case in monochrome tubes. Actuallythe principal eifect of the divergence of the beam is to make any shadowwhich the second grid wires might otherwise cast upon the screen lessprominent.

It should be evident that there is a rather wide range of structureswhich may be used in accomplishing the purposes of this invention.Others have shown methods of manufacturing grid structures of varioustypes; this specification is not concerned with the method ofconstructing the grids but rather with their positioning, and anysatisfactory structure can be used. Mention has already been made of thefact that the screen may be mounted inside of the. tube window in thesame manner as planar screens since the curvature of the screen is soslight. If the approximate type of correction is used, with acylindrical screen curved in only one dimension, the screen may beformed of plate, heated and bent in known manner. If the screen iscurved bidimensionally, as in the preferred form, it may be molded intothe required shape, and later sealed to the body of the envelope. Ifthis latter is the procedure chosen, it is convenient to curve thelonger edges of the approximately rectangular glass window downward,outside of the target area, so thatthe edge which is to be sealedto theenvelope lies substantially in a single plane. Although, for tubes whichare to be used with the NTSC system of standards of televisiontransmission it is preferred that the color control grid electrodes runparallel to the direction of line scanning rather than normal thereto,the same principles can be used with the color control grid electrodesrunning normal to the scanning lines. The details of constructionmentioned are therefore not intended to limit the scope of the inventionherein presented, all intended limitations being expressed in thefollowing claims.

Having now described the invention, what is claimed 1. In a cathode-raytube for the display of television images in color which comprises anevacuated envelope including a window area through which the images maybe viewed and an electron gun within said envelope for developing a beamof cathode ray directed toward said window area and adapted to bebidimensionally deflected thereacross to trace a raster; a target andcolor control structure comprising a substantially planar gridpositioned adjacent to said window area and approximately equal theretoin size, said grid having a multiplicity of apertures therein closelyand substantially uniformly spaced over substantially the entire area ofsaid grid, a second grid of like character positioned between said firstgrid and said window area in a plane substantially parallel to saidfirst grid, terminals external to said envelope connecting respectivelyto said grids for applying different electrical potentials thereto, anda display screen mounted in said window area and comprising atransparent base, a coating on said base comprising phosphors emissiveon electron impact of light of different colors, said phosphors beingdisposed in groups electro-optically alined with corresponding aperturesof said first grid and forming a repeating pattern coveringsubstantially the entire area of said screen, each group being in atleast one dimension of the order of magnitude of one elemental area ofthe images to be reproduced and including all of said phosphors, aconducting film disposed over said coating and means for connecting saidsecond grid to said conducting film, said base being concavely curvedtoward said electron gun.

2. In a cathode-ray tube for the display of television images in colorwhich comprises an evacuated envelope including a window area throughwhich the images may be viewed and an electron gun within said envelopefor developing a beam of cathode rays directed toward said window areaand adapted to be bidimensionally deflected thereacross to trace araster; a target and color control structure comprising a substantiallyplanar grid positioned adjacent to said window area and approximatelyequal thereto in size, said grid comprising a plurality of linearelectrodes closely spaced to form a multiplicity of aperturestherebetween substantially uniformly spaced over substantially theentire area of said grid, a. second grid of like character positionedbetween said first grid and said window area in a plane substantiallyparallel to said first grid, and with its linear electrodes running in adirection substantially normal to that of the electrodes of said firstgrid, terminals external to said envelope connecting respectively tosaid grids for applying different electrical potentials thereto, and adisplay screen mounted in said window area and comprising a transparentbase, a coating on said base comprising strips of phosphors emissive onelectron impact of light of difierent colors, said phosphor strips beingdisposed substantially parallel to the electrodes of said first grid ingroups forming a repeating pattern covering substantially the entirearea of said screen, the width of each group being of the order ofmagnitude of one elemental area of the images to be reproduced andincluding all of said phosphors and each group being electro-opticallyalined with a corresponding aperture of said first grid, a conductingfilm disposed over said coating and means for connecting said secondgrid to said conducting film, said base being concavely curved towardsaid electron gun.

3. In a cathode-ray tube for the display of television images in colorwhich comprises an evacuated envelope including a window area throughwhich the images may be viewed and an electron gun within said envelopefor developing a beam of cathode rays directed toward said window areaand adapted to be bidimensionally deflected thereacross to trace araster; a target and color control structure comprising a substantiallyplanar grid positioned adjacent'to said window area and approximatelyequal thereto in size, said grid comprising a plurality of linearelectrodes closely spaced to form a multiplicity of aperturestherebetween substantially uniformly spaced over substantially theentire area of said grid, a second grid of like character positionedbetween said first grid and said window area in a plane substantiallyparallel to said first grid and with its linear electrodes running in adirection substantially normal to that of the electrodes of said firstgrid, terminals external to said envelope connecting respectively tosaid grids for applying different electrical potentials thereto, and adisplay screen mounted in said window area and comprising a transparentbase, a coating on said base comprising strips of phosphors emissive onelectron impact of light of different colors, said phosphor strips beingdisposed substantially parallel to the electrodes of said first grid ingroups forming a repeating pattern covering substantially the entirearea of said screen, the width of each group being of the order ofmagnitude of one elemental area of the images to be reproduced andincluding all of said phosphors, and each group being substantiallyelectro-optically .alined with a corresponding aperture of said firstgrid, a conducting film disposed over said coating and means forconnecting said second grid to said conducting film, said base beingconcavely curved toward said electron gun, the depth of the concavityand the curvature thereof being such that for at least one componentangle of deflection in the direction normal to the electrodes of saidfirst grid, application of a potential difierence between said gridswhich will bring electrons entering said apertures to a focus at thesurface of said screen at one component angle of deflection in thedirection parallel to the electrodes of said first grid will bringelectrons substantially to a focus at the surface of said screen at anyother component angle of deflection in said last mentioned direction.

4. In a cathode-ray tube for the display of television images in colorwhich comprises an evacuated envelope including a window area throughwhich the images may be viewed and an electron gun within said envelopefor developing a beam of cathode raysdirected toward said window areaand adapted to be bidimensionally deflected thereacross to trace araster; a target and color control structure comprising a substantiallyplanar grid positioned adjacent to said window area and approximatelyequal thereto in size, said grid comprising two interleaved and mutuallyinsulated sets of elongated linear conductors, adjacent conductors beingsubstantially uniformly spaced over substantially the entire area ofsaid grid to form a multiplicity of .apertures therebetween, a secondgrid positioned between said first grid and said window area in a planesubstantially parallel to said first grid, and comprising elongatedlinear conductors disposed in a direction substantially normal to thatof the conductors of said first grid, terminals external to saidenvelope connecting respectively the sets of conductors of said firstgrid and to said second grid, for applying dilferent electricalpotentials thereto, and a display screen mounted in said window area andcomprising a transparent base, a coating on said base comprising stripsof phosphors emissive on electron impact of light of diflerent colors,said phosphor strips being disposed in groups forming a repeatingpattern covering substantially the entire area of said screen, the widthof each group being of the order of magnitude of one elemental area ofthe images to be reproduced and including all of said phosphors, andeach group being substantially electro-optically alined with acorresponding aperture of said first grid, a conducting film disposedover said coating and means for connecting said second grid to saidconducting film, said base being concavely curved toward said electrongun, the curvature and concavity of said screen being such that withrespect to at least one aperture of said first grid the application ofpotentials to said grid and screen which will bring electrons enteringsaid one aperture at any one angle of incidence to a focus at saidscreen will also bring electrons entering said aperture at any otherangle of incidence to a focus at said screen.

5. In a cathode-ray tube for the display of television images in colorwhich comprises an evacuated envelope including a window area throughwhich the images may be viewed and an electron gun within said envelopefor developing a beam of cathode rays directed toward said window areaand .adapted to be bidimensionally deflected thereacross to trace araster; a target and color control structure comprising a substantiallyplanar grid positioned adjacent to said window area and approximatelyequal thereto in size, said grid having a multiplicity of aperturestherein closely and substantially uniformly spaced over substantiallythe entire area of said grid a second grid of like character positionedbetween said first grid and said window area in a plane substantiallyparallel to said first grid, terminals external to said envelopeconnecting respectively to said grids for applying different electricalpotentials thereto, and a display screen mounted in said window area andcomprising a transparent base, a coating on said base comprisingphosphors emissive on electron impact of light of difierent colors, saidphosphors being disposed in groups electro-optieally alined withcorresponding apertures of said first grid and forming a repeatingpattern covering substantially the entire area of said screen, eachgroup being in at least one dimension of the order of magnitude of oneelemental area of the images to be reproduced and including all of saidphosphors, a conducting film disposed over said coating and means forconnecting said second grid to said conducting film, said base beingconcavely curved toward said electron gun and its concavity andcurvature being such that the coated surface of said base substantiallycoincides with a surface defined by the face of the electron lensesformed by said grids when a potential difiference is applied theretosuch as to bring the focus of any of said electron lenses upon saidsurface.

6. In combination with a cathode-ray tube comprising an evacuatedenvelop and an electron gun adapted to direct a beam of electronsagainst a target area within said envelop, said beam being deflectableto fall or any portion thereof, a target structure positioned withinsaid target area and comprising a cylindrically curved display screen, acoating comprising a repeating pattern of strips of phosphors emissiveupon electron impact of light of different colors additively producingwhite deposited on said screen in a direction normal to planes includingthe cylindrical axis of said screen, a grid of parallel conductorstautly supported across chords of said cylindrical screen 6 and securedclosely adjacent to the edges thereof, and a second grid of parallelconductors tautly supported in a plane substantially parallel to theplane of said first mentioned grid, the conductors of said second gridbeing parallel to and electro-optically alined with strips of saidcoating.

'7. In combination with a cathode-ray tube comprising an evacuatedenvelop and an electron gun adapted to direct a beam of electronsagainst a target area within said envelop, said beam being deflectableto fall on any portion thereof, a. target structure positioned withinsaid target area and comprising an electron lens structure comprising apair of spaced parallel grids, each grid comprising a multiplicity ofparallel linear conductors, the conductors of the two grids extending inmutually perpendicular directions, connections for establishingdifferent electrical potentials on said grids, a curved display screenmounted closely adjacent to the one of said grids more distant from saidelectron gun and a coating on said screen of light emissive phosphors,the curvature of said screen being substantially such that said phosphorcoating lies on a surface defined by the foci of the electron lensesformed by said grids with respect to electrons from said gun whendirected to the various areas of said electron lens structure.

8. In combination with a cathode-ray tube having an electron gun,including a cathode and an electron accelerating anode, directed towarda target area within an evacuated envelop, a target structure mountedwithin said area comprising a first grid and a second grid mounted insubstantially parallel planes separated by a distance D, each of saidgrids comprising a multiplicity of substantially parallel linearconductors and the conductors of said second grid extending in adirection substantially normal to that of the conductors of said firstgrid, connections for applying diiferent voltages between said cathodeand said first grid and said first and second grids, a curved displayscreen mounted closely adjacent to said second grid, and a coating ofphosphors emissive of light on electron impact deposited on said displayscreen, the curvature of said display screen being such that itsdistance F from the plane of said second grid bears to the distance Dthe ratio 2 /I+K see? 0 1+tan a tan 5 K sec. 6 1+K sec. 0

FID=

References Cited in the file of this patent UNITED STATES PATENTS Re.23,672 Okolicsanyi June 23, 1953 2,606,246 Sziklai Aug. 5, 19522,669,675

Lawrence Feb. 16, 1954

